On buses and billboards across Jerusalem, a poster depicts three men not commonly associated with each other - US President George Bush, Hamas leader Ismail Haniya and Hezbollah leader Hassan Nasrallah.

But what does George Bush have in common with two men he must regard as arch enemies?

The tagline of the poster makes it clear - "Bush, free your captive!"

Hamas has been holding an Israeli soldier captive in Gaza since 2006, while Hezbollah captured two Israeli soldiers.

The "captive" that Mr Bush is being asked to set free is Jonathan Pollard.

The case of Pollard, a US Navy intelligence analyst who gave classified material to Israel, is a sore point in otherwise excellent relations between Israel and the US.

'Bargaining Chip'

In 1987, Pollard was sentenced to life imprisonment without parole.

Mrs Pollard has written to Mr Bush pleading for clemency

The Israelis publicly admitted that Pollard was their agent in 1998 and awarded him Israeli citizenship.

During Mr Bush's three-day visit to Israel and the West Bank, Pollard's supporters are once again demanding his release.

Along with the poster campaign and small rallies, his wife, Esther, has written a letter to the US president pleading for clemency.

Pollard's supporters argue that he could not have committed "treason" as both Israel and the US are allies. They say that Pollard fully co-operated with a subsequent investigation, only to receive an unduly harsh sentence.

Mrs Pollard says that the refusal to release her husband is politically motivated and that the US is using him as "bargaining chip".

"At every single juncture my husband's plight is exploited for political gain," she told BBC News.

"Both the US administration and the state department are very aware of his value to the Israeli public."

'Traitor'

The documents that Pollard leaked have never been made public for intelligence reasons. But they were said to include information on Soviet arm shipments to Syria, Iraqi and Syrian chemical weapons, the Pakistani atomic bomb and Libyan air defences, according to the author and journalist Wolf Blitzer in his book Territory of Lies.

We have to assume that not only did the intelligence go to Israel but that it went to other countries as well

NIS agent Ron Olive

While Pollard's supporters seek to play down his actions, many American officials regard him as a traitor and as a man who seriously damaged US security interests.

Ron Olive, the agent in charge of counterintelligence for the Naval Investigative Service at the time of Pollard's arrest, told the BBC that the incident was "one of the most devastating cases of espionage in US history" during which Pollard stole over "one million classified documents".

"Even though Israel is an ally, it had friends that aren't necessarily friend of the US," said Mr Olive, who wrote a book about the case, Capturing Jonathan Pollard.

He argues that all countries' intelligence services are susceptible to penetration, including Israel.

"We have to assume that not only did the intelligence go to Israel but that it went to other countries as well," he added.

While many Israeli officials would like to forget about the Pollard affair, the issue is periodically raised at the highest levels.

Presidential pardon

In 1998, former Israeli Prime Minister Benjamin Netanyahu was reported to have won a guarantee from then-President Bill Clinton that Pollard would be released, tied to a Palestinian prisoner exchange deal.

But the CIA boss at the time, George Tenet, threatened to resign if President Clinton agreed to the release.

If the Pollard affair was a low point in Israeli-American relations, the damage it did at the time seems to have been forgotten by the two sides.

Mr Bush was hailed as something of a prodigal son in Israel.

"I think it's clear from this visit that the Pollard case is not an issue and that Israeli-American relations are as strong as ever," says Gabriel Sheffer, a professor of politics at Hebrew University.

But Pollard's supporters say they will not stop fighting until he is released.

Mrs Pollard says she is hopeful of a possible US presidential pardon. "I'm hopeful that we're reaching the end of the time," she said.